An old device called a stereopticon held two photographs taken from closely related viewpoints, such that on looking into it the observer saw a three-dimensional view of the photographed scene. This proves that we unconsciously construct, in our brains, a three-dimensional space out of two two-dimensional images, one per retina. Also, if you have someone hold up a finger, it is easy to bring your finger down on to its tip, but if you try this with one eye closed it is difficult -- proving that two eyes are necessary for seeing three-dimensional space. But this means that our three-dimensional visual space is inside our heads, whereas we clearly experience it as out side our heads. So which is it?